ARM: samsung: don't build plat/pm-common for Exynos

This file has not had any callers since linux-4.7, so
stop building it.

Fixes: 5901f4c2 ("ARM: EXYNOS: Remove SROM related register settings from mach-exynos")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20200806182059.2431-10-krzk@kernel.org


Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>